I'm don't understand how a phase anomaly would prevent the tone control from affecting the sound, though I can see how it might remove some of the bass frequency, which is also part of the problem.

More confused than ever

Rich

It completely explains the (otherwise incomprehensible) chip choice, and I'd be very interested to know what exactly they are doing to get a supposed 'stereo' effect. Oh, and poor LF response could be simply down to caps drying out.

I'm a bit suspicious of the tone control doing nothing at all. A simple fault would suggest only one side being affected. Also, no stereo effect at all sounds a bit strange. I wouldn't be surprised if it turned out to be something daft like a hairline fracture in a ground track.

This is proving to be a very strange design indeed! I've worked out a drawing, and it's so strange I had to keep re-checking it See here.

(Updated diagram shown above)Cold checks with a meter suggest there are no faults, so the tone control should work, if there are no dried out caps, although with such low impedances I'm not quite sure how! The unmodified gain of the amplifers is 20, so those 1u feedback caps will behave as if they were about 20u.

Notice that DC supply to the microphones. This makes me think they are electrets

As soon as i can get into the office, I'll snaffle their all-singing cap tester and find out what the actual values are.

The 150n 'dry' caps are a guess based on the amp spec sheet.

The 0u22 ones are actually mis-printed so any dot would be off the edge of the cap. Size-wise they couldn't realistically be 22u, and it wouldn't make sense, bearing in mind the feed caps are only 33u.

Also today I did a few active tests in my {cough}kitchen{cough} workshop, using my signal generator (of DIY amp fame) and a modern true RMS multimeter. At these low frequencies they're pretty good.

I worked with an output level of 1V, which is quite reasonable as a headphone level, and as I'm not sure of the correct source impedance, I used 600ohms, 2.2k and 47k. The result was entirely consistent across all of these, which is something I didn't expect actually.

I can confirm the tone control has no measurable effect at all over the range 40Hz to 4kHz

Also, the -3dB point relative to 400Hz is 70Hz, which is way too high for a bass.

I might try tapping caps across those present (particularly those 33u coupling ones) and see what transpires.

I'm not at work tomorrow, but if I'm down that way I might drop in and see if I can borrow the cap tester - I should probably get one myself

OK, there is an error in tracing the circuit, and it now makes much more sense

(Updated image -- HR)

If you click on the link itself, you'll get the corrected drawing. The 1u caps are compensation ones that go from the top of the volume control to the wiper.

This doesn't invalidate any of the actual tests. Also the latest news is that the 0.47u caps are dead, tapping one across them immediately made the tone control work.

The 'dry' cap and the 33u one are effectively forming a 2nd order high pass filter, both of them are dropping a significant voltage at 70Hz. Using an R/C calculator, the 33u one against the volume pot at full volume should give a -3dB point of 4.8Hz! so it looks like those are also dead-ish and the only ones that's supposed to be affecting the roll off should be the 'dry' ones. I only guessed their values, but with the behavior shown and good caps elsewhere I would expect the -3dB point to be nearer 45Hz - much more reasonable.

I'm wondering if the unit has been left somewhere hot for an extended period, or is much older than I thought.

Pretty sure they are going to be electret mics given the DC supply present on the mic lines. All electrets have an active impedance converter-cum-output-driver. So they will have a very low output impedance of the order of 100 Ohms or less.